The film was completed in 2009, but release was postponed until 2012. The official explanation was because Lionsgate wanted to spend an extra year or two post-converting the film into 3D. It was also speculated that the film was postponed partially because actor Chris Hemsworth and co-writer Joss Whedon would subsequently work on several films in the "Marvel Cinematic Universe" (with Hemsworth starring in "Thor" and "The Avengers", and Whedon directing "The Avengers"), and Lionsgate was hoping that their increased public awareness would boost box-office numbers and DVD/Blu-Ray sales.
Fran Kranz, who plays stoner Marty, had to wear baggy clothing for his role in the film, as the filmmakers realized he was incredibly fit, and didn't quite look the part of the lanky, somewhat meek stoner he was playing. The scene where the characters go swimming at the lake actually had to be re-written so his character wouldn't go swimming as originally planned, as Franz was in such good shape he looked as good, if not better, than the other male actors.
One of the creatures glimpsed jumping out of the lift is one of the small monsters which attack people in the subway in "Cloverfield". The director of this movie, Drew Goddard, wrote Cloverfield.
The movie pays tribute to many famous horror movie monsters. The creature with the Buzz-Saws in its head with the Puzzle Orb is based off PinHead. Once it's free, you can see a Chatterbox-influenced cenobyte standing next to it. The people with ceramic masks who tie up and torture the scientists are based off The Strangers. The psychotic clown with the knife can either be seen as a tribute to PennyWise or Killer Clowns from Other Space. The massive killer bat is obviously from the movie BATS. A chainsaw can be heard as the different monitors are showing the slaughter - indicating LeatherFace. The ghoul-doctors operating on a victim while staring into the camera is based off House on Haunted Hill. The Red-Neck zombies brought to life through an incantation from a book is a tribute from The Evil Dead. The Scare-Crow-like zombies who kill the black security guard are based off of the movie ScareCrow.